1. Field of the Invention
The present invention relates to the field of structured optical films and optical displays incorporating the structured optical films.
2. Discussion of Related Art
Structured optical films are used in optical display systems and in other applications where control over the direction of light, transmitted and/or reflected, is desired to increase brightness, reduce glare, etc. Structured optical films are described in U.S. Pat. No. 4,906,070 (Cobb). Essentially, they comprise films of light transmissible materials on which a series of prisms is disposed such that the films can be used to redirect light through reflection and refraction. When used in an optical display such as that found in laptop computers, watches, etc., the structured optical film can increase the brightness of the optical display by limiting light escaping from the display to within a desired viewing range, for example defined by a pair of planes disposed at desired angles from a normal axis running through the optical display. As a result, light that would otherwise exit the display outside of the allowable range is reflected back into the display where a portion of it can be “recycled” and returned back to the structured film at an angle that allows it to escape from the display. Such recycling is useful because it can reduce the power consumption needed to provide a display with a desired level of brightness.
FIGS. 1 and 2 generally illustrate the concept of structured optical films. FIG. 1 depicts a section of a regular, simple repeating pattern structured optical film 10 including a structured surface 12 and a planar surface 14. The structured surface includes a series of regularly spaced valleys 16 and peaks 18 that define prisms 20. The prisms 20 are defined by facets formed between the valleys 16 and peaks 18. The geometry of the structured surface 12 and the material used to manufacture the film 10 foster total internal reflection and refraction of light entering the planar side 14 of film 10 to minimize the escape of light through the structured surface outside of the desired range of angles.
Some optical displays incorporate a second structured optical film in which the prisms are oriented at an angle with respect to the prisms in the first optical film. That angle can be anywhere from greater than zero to 90°, although it is typically about 90°. FIG. 2 illustrates a pair of structured optical films 22 and 24 in which the prisms 26 and 28, respectively, are oriented at approximately a 90° angle with respect to each other. In use, it is preferred that the structured surface 28 be in contact with, or nearly in contact with, the planar surface 27 of the upper film 22.
An undesirable effect of using a structured optical film in an optical display is the appearance of reflected moiré effects caused by the interference of two superimposed repeating patterns. Moiré effects are discussed in “The Theory of the Moiré Phenomenon”, by Isaac Amidror, Kluwer Academic Publishers, Dordrecht, The Netherlands (2000). In an optical display incorporating a single layer of structured optical film, the repeating patterns causing moiré effects are the pattern in the film itself and the reflected image of the film pattern, as reflected by other surfaces in the optical display.
Although using two structured optical films can increase the brightness of the display within a narrowed viewing range, this can also increase the appearance of moiré effects by providing a second planar surface, on the lower structured film, that reflects more light back through the repeating pattern in the first, or upper, structured film.
Addressing moiré effects in brightness enhancement films, U.S. Pat. No. 5,919,551 (Cobb) describes the use of structured optical films in which the pitch of the peaks and/or valleys is different for successive groups of peaks and/or valleys. However, while such films apparently reduce the visibility of moiré effects, the brightness of an LCD display employing such films may be reduced by approximately 6% or more relative to LCD displays which use films as shown in prior art FIG. 2.
FIG. 3A schematically depicts a cross-section, normal to the planar surface 32, of one such structured optical film 30. The film 30 includes a set of prisms defined by peaks 36 and valleys 38. The peaks 36 and valleys 38 defining the prisms are substantially parallel to each other. The spacing between adjacent peaks 36, i.e., the peak pitch, of the optical film 30 is substantially constant. The spacing between adjacent valleys 38, however, varies over any group of three successive valleys 38. That spacing between valleys 38 can also be referred to as valley pitch, Pv. By varying the valley pitch, the visibility of moiré interference patterns may apparently be reduced when using film 30 in an optical display.
In the film 30 depicted in FIG. 3A, peak pitch is held constant while the valley pitch varies. The tooling used to manufacture the film 30 can, however, be replicated by electroforming or other suitable processes, thus forming a “negative” of the pattern formed on the master tool. When that replicated tool is then used to form a film, the result, depicted in FIG. 3B from the prior art, is a film 130 that is a “negative” of the film 30 depicted in FIG. 3A. As a result, film 130 has a constant valley pitch, Pv, between valleys 136 while the peak pitch, Pp, between peaks 138 varies across the film 130. That is exactly the opposite of the pitch characteristics of the film 30. Like film 30, the film 130 in FIG. 3B may also apparently be used for reducing the visibility of moiré interference patterns when used in an optical display.
However, even with the ‘grouped’ structures described above, it should be understood that moiré interference effects will be observable under certain circumstances. The reason is that the structural arrangement of the film described above still retains the capacity to produce moiré interference effects.
In addition, the use of a second structured optical film may also lead to optical coupling between the two films. This can result in uneven light transmission from the display, i.e., visible bright spots, streaks, and/or lines in the display. Optical coupling is caused by contact, or very near contact, between the structured surface of a structured optical film and a planar surface disposed above the film.
U.S. Pat. No. 5,771,328 (Wortman) describes the use of structured optical films with a variable height in order to reduce the influence of optical defects such as bright spots, streaks and lines in display applications. FIGS. 4-6 illustrate representative prior art constructions for such a light directing film. It should be noted that these drawings are not to scale and that, in particular, the size of the surface is greatly exaggerated for illustrative purposes.
However, it should be understood that the films disclosed in U.S. Pat. No. 5,771,328 still suffer from the moiré interference effects described above, due to their structural arrangement.
Films in which the valley dihedral angles vary from valley to valley are known from WO1997/28468. Films in which the peak and valley dihedral angles vary from peak to peak and from valley to valley are known from WO2002/23258 and WO2001/27663. However, such variation in prism angle and orientation is undesirable, since may reduce the uniformity and controllability of light redirection in the films.
A linear prismatic surface comprising peaks with an irregular spacing is disclosed in JP06-082635. However, the structure of the surface is complex, and not easy to manufacture.